During the development of the mammalian brain, neuronal precursor fate is specified by localized positional information. Little is known about the genes that specify these regional differences. The goal of this project is to identify genes in the Drosophila brain that function early in the formation of the visual system to specify the precursors of the optic ganglia. Genes identified in the fly might then serve as probes for identifying cognates that have similar functions during mammalian brain development. Using classical genetic and molecular genetic approaches available in the fruit fly, a number of genes have been identified whose expression patterns in the larval brain suggests they play an early role in visual system development. The current focus of this project is: (1) the analysis of an enhancer element that activates transcription in the first optic ganglia, the lamina, in response to axon ingrowth from the developing retina; and (2) genetic and molecular analysis of the mutant Drop, a gene involved in controlling cell proliferation in the visual system.